Low density fillers consisting of synthetic hollow spheres have recently been marketed for use in filling or extending various polymer systems including thermoset plastics such as unsaturated polyesters and epoxies. Such spheres are especially useful for incorporation into unsaturated polyester resin systems which are used to prepare simulated wood. They are also useful as fillers for polyesters in spray-up applications to prepare white ware such as bath tubs and sinks.
The hollow spheres utilized for this purpose are generally prepared by spray drying a solution of a film-forming material in a volatile solvent at elevated temperatures, thereby forming hollow spheres which are recovered from the spray dryer. These spheres may be further dried by heating, vacuum treatment or exposure to dry air to remove solvent and set the walls. A wide variety of film forming materials such as those discussed in U.S. Pat. No. 2,797,201 can be used to produce such spheres. A system of particular value in our invention is disclosed in U.S. Pat. No. 3,796,777. This patent discloses the spray drying of a solution containing sodium silicate and a polysalt to form small hollow spheres useful as fillers in plastics.
The polymer systems in which these hollow spheres are used may often require other additives such as pigments or fire retardants. Processing of such systems may also require that the density of the hollow sphere filler approximate the density of the polymer system. We have now discovered a method to modify such fillers to provide properties which meet these requirements.